1. Field of the Invention
The present invention relates to a signal amplifying circuit, and in particular to a signal amplifying circuit connected to a transfer circuit having a non-linear transfer characteristic and a transient characteristic.
Generally, signal transfer circuits or transmission lines may have a non-linear transfer characteristic. This non-linear transfer characteristic is positively used in an optical receiving circuit of an optical subscriber system, and is indispensable for extending the dynamic range of an amplifying circuit. In addition, a preamplifier for converting an optical signal into an electric signal has a "0" level rise peculiar to an optical signal transmission and a tailed response characteristic as a transient phenomenon. The signal amplifying circuit connected to the preamplifier is required to deal with such a response characteristic.
2. Description of the Related Art
FIG. 42 shows an example of a prior art signal amplifying circuit (1) having a preamplifier 20 with a non-linear transfer characteristic at the former stage. In the preamplifier 20, after having been converted into an input current signal I by a photo diode 10, a burst optical input signal 100 of an optical subscriber system is inputted to an amplifier 21 having a feedback resistor 22 and a diode 23 with a non-linear characteristic.
If the direction of an arrow indicating the current signal I in FIG. 42 is assumed to be positive, a signal 101 which is the output signal of the amplifier 21 has a negative logic. Also, the optical input signal 100 from subscribers comprises burst signals whose amplitudes are largely different from each other. The preamplifier 20 and a signal amplifying circuit 30 are required to instantaneously respond at the leads of the burst signals (in a wide range).
The signal amplifying circuit 30 is composed of a threshold generation circuit 33 for receiving the input signal 101 from the preamplifier 20 and outputting a threshold signal 106, and a limiter amplifier 31 which is a differential input/output amplifying circuit for inputting the input signal 101 and the threshold signal 106 and outputting output signals 102 and 103.
The threshold generation circuit 33 is composed of a peak detection circuit 34 and a bottom detection circuit 35 for commonly inputting the input signal 101, and a voltage divider 36 for inputting a peak detection signal 104 and a bottom detection signal 105 which are the outputs of the detection circuits 34 and 35 and outputting the threshold signal 106 having a partial voltage between a peak level and a bottom level.
FIGS. 43A and 43B show the amplifying characteristics of the preamplifier 20 and the signal amplifying circuit 30. The circuit operation of FIG. 42 will now be described referring to FIGS. 43A and 43B.
FIG. 43A shows the transfer characteristic A of the preamplifier 20, that is the relationship between the input current signal I and the output amplitude (=the amplitude of the input signal 101), and shows a threshold level L1 set by the threshold generation circuit 33 of the signal amplifying circuit 30, that is the relationship between the input current I and the threshold signal 106.
The transfer characteristic A reveals a linear characteristic determined by a feedback resistor 22 in the range from the input current 0 to I1, and reveals a compressed curved line with a non-linear characteristic of the diode 23 further added in the range over the input current I1.
This is because the preamplifier 20 prevents the output signal resistance including the resistance of the diode 23 to enhance the negative-feedback effect in order to extend the dynamic range of the optical input signal 100 when the preamplifier 20 receives the optical input signal 100 having an excessive amplitude.
The threshold level L1 indicates the level of the threshold signal 106 which corresponds to a partial voltage between the peak detection signal 104 and the bottom detection signal 105 which are respectively detected by the peak detection circuit 34 and the bottom detection circuit 35 with respect to the input signal 101 from the preamplifier 20, the partial voltage being provided by resistors 41 and 42 having the same resistance as an example. Accordingly, the threshold level L1 is a half level of the amplitude of the input signal 101 shown by the transfer characteristic A.
FIG. 43B shows a pulse width B of the input signal 101 outputted by the preamplifier 20 and a pulse width C of the output signals 102 and 103 outputted by the signal amplifying circuit 30 with respectively being made correspond to the amplitude of the input signal 101, assuming that the pulse width of the optical input signal 100 has 100% (a single time slot).
It is to be noted that the amplitude of the input signal 101 indicates the difference between the peak level and the bottom level of the signal 101, and the pulse width of the input signal 101 indicates a signal width at an intermediate level between the peak level and the bottom level of each signal according to the above-mentioned example.
The pulse width B is the same as that of the optical signal 100 when the amplitude of the input signal 101 is small, while the pulse width B greatly varies from a vicinity of a control initiation level V1 of the amplitude due to the non-linear transfer characteristic of the preamplifier 20. Since the signal amplifying circuit 30 which inputs a signal having the pulse width B performs a linear operation, it generates the output signals 102 and 103 which have the pulse width C substantially the same as the pulse width B.
FIGS. 44A and 44B show an example of an operation waveform in the operation of the signal amplifying circuit 30. FIG. 44A shows an example in which the amplitude of the optical input signal 100 is small, so that the preamplifier 20 linearly amplifies the optical input signal 100. Accordingly, the pulse width of the input signal 101 is the same as that of the optical input signal 100.
The signal amplifying circuit 30 outputs the output signals 102 and 103 based on the threshold signal 106 which has an intermediate level between the peak detection signal 104 and the bottom detection signal 105 of the input signal 101. Accordingly, the pulse width of the output signals 102 and 103 is substantially the same as that of the optical input signal 100.
FIG. 44B shows an example in which the amplitude of the optical input signal 100 is large, so that the preamplifier 20 compresses and amplifies the side of logic "1" of the optical input signal 100 to output the signal 101 deteriorated in the direction of widening the pulse width as shown by a dotted line. Accordingly, the pulse width of the output signals 102 and 103 the signal amplifying circuit 30 outputs based on the threshold signal 106 which has an intermediate level between the peak detection signal 104 and the bottom detection signal 105 is larger than the pulse width of the optical input signal 100.
Also, there is a problem on the level of the threshold signal 106 generated by the threshold generation circuit 33 that the pulse width is further deteriorated. Therefore, a necessary eye pattern (aperture) can not be obtained especially when the input signal is small because the level of the threshold signal 106 deviates from the central position of the signal due to the offset or the like.
In order to solve this problem, measures such as correcting the pulse width by using an average value of the input signal have been taken when the input signal is a continuous transmission signal. However, since an average value circuit such as an LPF is slow to respond, it is impossible to apply the LPF for the transmission of the burst signal which requires an instantaneous response at the lead of the input signal.
In such a prior art signal amplifying circuit, there has been a problem that the pulse width varies and deteriorates due to the non-linear transfer characteristic of a transfer circuit at the former stage.
Also, in an optical burst transmission, for instance, there are two major problems due to a transient response generated at the lead of the cell.
One of them is a tailed waveform due to a low frequency response of a photo diode (a photo device), and the other is a "0" level rise due to an optical extinction ratio deterioration in the signal.
The tailed waveform will now be described. FIG. 45 shows a frequency response characteristic efficiency of a photo diode. The ordinate and the abscissa respectively indicate efficiencies and frequencies. The frequency response characteristic has a shoulder portion in the range of several kHz to several hundred kHz, so that the tailed waveform is generated in the presence of the shoulder portion.
FIGS. 46A and 46B show an example of operation waveform when burst cell signals whose optical powers are mutually different sequentially arrive at the photo diode 10 shown in FIG. 42. The photo diode 10 has the frequency response characteristic of FIG. 45.
FIG. 46A shows the optical input signal 100 (see FIG. 42), in which a packet P2 of small optical power arrives after a packet P1 of large optical power. The current signal I (see FIG. 42) of the photo diode 10 at that time is shown in FIG. 46B. The "0" level of the packet P1 rises due to the low frequency response, which remains up to the lead of the following packet P2, resulting in a tailed waveform (see the dotted circle).
FIG. 47 shows an example of a prior art signal amplifying circuit (2). This signal amplifying circuit 30 is different from that shown in FIG. 42 in that basic amplifying circuit blocks 30_1 and 30_2 which have the same arrangement as the signal amplifying circuit 30 shown in FIG. 42 are connected in a multistage form. This is because the multistaged signal amplifying circuit enables a level variation at the first stage to be corrected at the second and the following stages.
The packet P2 of FIG. 46B amplified at the amplifier 21 and a buffer 24 corresponds to an input signal 101_1. An example of an operation waveform (1) when the input signal 101_1 is inputted to the signal amplifying circuit 30 is shown in FIGS. 48A, 48B, and 48C. FIG. 48A shows an operation waveform as to a threshold generation circuit 33_1 which inputs the input signal 101_1 with a tailed waveform.
After having detected the peak level of the input signal 101_1, a peak detection circuit 34_1 provides a peak detection signal 104_1 holding the level since the input signal 101_1 does not exceed the peak level. On the other hand, since the input signal 101_1 has a tailed waveform, a bottom detection circuit 35_1 provides a bottom detection signal 105_1 which sequentially detects the bottom level of the input signal 101_1. A threshold signal 106_1 is at an intermediate level between the peak detection signal 104_1 and the bottom detection signal 105_1.
A limiter amplifier 31_1 performs a differential amplification between the input signal 101_1 and the threshold signal 106_1 to output an input signal 101_2 of the basic amplifying circuit block 30_2 at the next stage. Since the input signal 101_1 is small, the differential amplification is performed in a linear form, so that the input signal 101_2 which is the output signal of the circuit 30 has a tailed waveform without saturations.
FIG. 48B shows an operation waveform in a threshold generation circuit 33_2 which inputs the input signal 101_2. A peak detection signal 104_2, a bottom detection signal 105_2, and a threshold signal 106_2 respectively show the peak and the bottom levels of the input signal 101_2 and the intermediate level between the peak and the bottom level in the same way as the threshold generation circuit 33_1.
A limiter amplifier 31_2 performs a differential amplification between the input signal 101_2 and the threshold signal 106_2 to provide the output (positive) signal 102 and the output (negative) signal 103, while the limiter amplifier 31_2 operates as a limiter since the amplitude of the input signal 101_2 is large.
FIG. 48C shows a waveform of the output signal 102 and the output signal 103. As shown in FIGS. 48A and 48B, since the threshold signals 106_1 and 106_2 are respectively set to a higher level than the intermediate level of the amplitude of the input signals 101_1 and 101_2, the pulse width of the output signal 102 and the output signal 103 becomes small. This phenomenon becomes more remarkable as the tailed waveform of the input signal 101_1 becomes large. When the threshold signal level 106_1 (106_2) exceeds the input signal 101_1 (106_2), the output signals 102 and 103 will not vary.
A laser diode (LD) can improve an output waveform (the optical input signal 100 of FIG. 47) by flowing therethrough a bias current, which leads to another problem, i.e. the optical extinction ratio deterioration.
This problem will be described as follows:
In FIG. 47, the photo diode 10 converts the optical input signal 100 whose optical extinction ratio is e.g. 10 dB into the current signal I, which is inputted to the amplifier 21 of the preamplifier 20. An input current I vs output voltage (=input signal 101_1) characteristic of the preamplifier 20 is shown in FIG. 49.
Having a non-linear function by the diode 23, the amplifier 21 in the preamplifier 20 outputs a "0" level bias due to the optical extinction ratio deterioration in the input current as a "0" level having a larger bias voltage.
FIGS. 50A, 50B, and 50C show an example of an operation waveform (2) in the case where the signal amplifying circuit 30 in FIG. 47 receives a signal having such a bias voltage as the input signal 101_1. FIG. 50A shows an example of an operation waveform of the threshold generation circuit 33_1. The peak detection signal 104_1 sequentially becomes large following the input signal 101_1, and maintains the level after the peak level of the input signal 101_1 is stabled.
On the other hand, the bottom detection signal 105_1 keeps the bottom level initially detected since the input signal 101_1 does not become smaller than the bottom level in a transition period. The threshold signal 106_1 is set to the intermediate level between the peak detection signal 104_1 and the bottom detection signal 105_1.
In this example, the threshold signal 106_1 is set below the input signal 101_1. Accordingly, the limiter amplifier 31_1 outputs the signal i.e. the input signal 101_2 fixed to the "1" level without reproducing the waveform of the input signal 101_1.
The operation waveform of the threshold generation circuit 33_2 which receives the input signal 101_2 is shown in FIG. 50B. The peak detection signal 104_2 assumes a state of keeping the "1" level voltage in the stationary state following the input signal 101_2, while the bottom detection signal 105_2 keeps the "0" level voltage of the initial input signal 101_2. The threshold signal 106_2 is set to the intermediate level between the peak detection signal 104_2 and the bottom detection signal 105_2, that is a level below the input signal 101_2.
Accordingly, the output signals 102 and 103 of the limiter amplifier 31_2 are respectively fixed to the "1" level, and can not reproduce the input signal 101_1. This is also recognized from the fact that the input signal 101_1 is not already reproduced in the limiter amplifier 31_1 at the former stage.
As means for solving the above-mentioned tailed waveform, a signal amplifying circuit using a master-slave type threshold generation circuit has been proposed by the inventors of the present invention in the Japanese Patent Laid-open No.10-261940. Namely, the bottom detection circuit detects the bottom level of the input signal, and the peak detection circuit detects a relative peak level to the bottom level of the input signal. As a result, the peak detection circuit can set an adequate threshold signal without keeping the peak level in the transition period.
However, for the input signal having both the tailed waveform and the "0" level rise, the peak detection circuit is required to detect a lower level than a transient highest value. At the same time, the bottom detection circuit is required to detect a higher level than a transient lowest value. However, as the polarities of transient responses are different, it has been difficult to solve both of the problems simultaneously.